Elongation factors belong to a family of proteins that promote the GTP-dependent binding of aminoacyl tRNA to the A site of ribosomes during protein biosynthesis, and catalyse the translocation of the synthesised protein chain from the A to the P site. The proteins are all relatively similar in the vicinity of their C-termini, and are also highly similar to a range of proteins that includes the nodulation Q protein from <taxon tax_id="382">Rhizobium meliloti</taxon> (Sinorhizobium meliloti), bacterial tetracycline resistance proteins [<cite idref="PUB00002069"/>] and the omnipotent suppressorprotein 2 from yeast. <p>In both prokaryotes and eukaryotes, there are three distinct types of elongation factors, EF-1alpha (EF-Tu), which binds GTP and an aminoacyl-tRNAand delivers the latter to the A site of ribosomes; EF-1beta (EF-Ts), which interacts with EF-1a/EF-Tu to displace GDP and thus allows theregeneration of GTP-EF-1a; and EF-2 (EF-G), which binds GTP and peptidyl-tRNA and translocates the latter from the A site to the P site. In EF-1-alpha, a specific region has been shown [<cite idref="PUB00000682"/>] to be involved in a conformational change mediated by the hydrolysis of GTP to GDP. This region is conserved in both EF-1alpha/EF-Tu as well as EF-2/EF-G and thus seems typical for GTP-dependent proteins which bind non-initiator tRNAs to the ribosome. The GTP-binding protein synthesis factor family also includes the eukaryotic peptide chain release factor GTP-binding subunits [<cite idref="PUB00001272"/>] and prokaryotic peptide chain release factor 3 (RF-3) [<cite idref="PUB00002900"/>]; the prokaryotic GTP-binding protein lepA and its homologue in yeast(GUF1) and <taxon tax_id="6239">Caenorhabditis elegans</taxon> (ZK1236.1); yeast HBS1 [<cite idref="PUB00000882"/>]; rat statin S1 [<cite idref="PUB00002632"/>]; and the prokaryotic selenocysteine-specific elongation factor selB [<cite idref="PUB00004050"/>].</p>